1. Technical Field
The present disclosure relates to a current output circuit which is connected to an external circuit through two transmission lines and outputs a predetermined current signal to the external circuit while using the external circuit as a power source, and a two-wire transmitter having the current output circuit.
2. Related Art
A two-wire transmitter is, for example, a field device which is connected to an external circuit such as a DCS (distributed control system) through two transmission lines and converts a physical quantity acquired from a sensor etc. into a current signal and outputs the current signal to the external circuit while using the external circuit as a power source. Since the two-wire transmitter does not require dedicated power source wiring and can be installed at low cost, the two-wire transmitter is widely used as the field device such as a temperature transmitter or a differential pressure and pressure transmitter in a plant. This field device converts a physical quantity into a direct current of 4 [mA] to 20 [mA] of a global standard as a signal of the field device, and sends the current to the external circuit.
For example, Patent Reference 1 discloses a two-wire transmitter capable of ensuring a stable circuit power source and freely setting an internal voltage by a shunt regulator (shunt voltage source circuit) rather than a zener diode.
FIG. 6 shows a configuration example of a related-art current output circuit 50 used in the two-wire transmitter described above. According to FIG. 6, the current output circuit 50 is configured to include a voltage source circuit 51, an operational amplifier 52 (op amp), a voltage-to-current conversion element 53 (NPN transistor), a current mirror circuit 54, a shunt voltage source circuit 55, a current detecting resistor R11, a feedback resistor R12, a band limiting resistor R13, and a band limiting capacitor C11. By the configuration described above, the current output circuit 50 outputs a current I1 by the shunt voltage source circuit 55 and also generates an internal power source #1 (a driving power source of a signal processing circuit or a sensor (not shown)) of the two-wire transmitter and outputs a current of Uout=(1+R12/R11)*I1 between a positive power source terminal VP and a negative power source terminal VN.